WASHINGTON (AP) -- President Donald Trump's personal assistant was removed from his job this week, leaving the president more isolated amid a growing exodus of many of his most trusted aides.

John McEntee, who served as the president's personal assistant, was escorted out Monday, according to two people close to the White House who spoke on condition of anonymity in order to discuss private conversations.

One of a shrinking group of former campaign aides still working in the West Wing, McEntee quickly rejoined the president's campaign. Trump's 2020 Republican campaign announced Tuesday that he will serve as a senior adviser for campaign operations.

White House spokeswoman Sarah Huckabee Sanders declined to comment on the reasons for his departure, saying only that McEntee was going to the campaign.

In response to news reports about McEntee's exit, Rep. Elijah Cummings, the ranking Democrat on the House oversight committee, sent a letter to chief of staff John Kelly requesting all documents related to McEntee's employment and departure.

"I have been warning for more than a year about the White House's deficient background check process - as well as specific officials who have been granted access to our nation's most closely guarded secrets despite derogatory information known to White House officials," wrote Cummings.

McEntee, 27, who was well liked in the West Wing, occupied a key role, never far from Trump in the White House or on the road. His exit comes after a number of top aides have left the building.

Trump announced Tuesday that he had ousted Secretary of State Rex Tillerson. Trump's top economic adviser, Gary Cohn, left recently amid internal debate over Trump's plans for tariffs. Communications director Hope Hicks also recently announced her departure.

Like Hicks, McEntee was part of a small group of aides that had been with Trump since his campaign. Their departures could leave Trump - who loves to reminisce about his winning campaign - feeling lonely.